The field of the invention is plasma generating processes and apparatus, and the invention relates more particularly to the use of plasma streams to desmear and etchback the openings in multi-layered resin/copper laminates.
The use of plasma to etchback the resin in holes through printed circuit boards is described in U.S. Pat. No. 4,012,307. A process is there-described utilizing an upper electrode positioned on the top of a chamber and a ground electrode positioned on the bottom thereof. A plurality of vertically disposed multi-layer printed circuit boards are placed in the chamber, and the chamber is evacuated and a gas capable of forming an oxidizing plasma is introduced into the reactor. This results in a flow of plasma in the chamber between the radio frequency electrode and the ground electrode which tends to etchback the resin portion of multi-layered boards exposing the copper layers in the openings of the board.
While such system is an improvement over the formerly used chemical dip process, control of the amount of etchback is deficient and processing times are unnecessarily long. The flow of plasma tends to heat the circuit boards which, thus, increase in temperature during processing which, in turn, increases the etch rate. Because the flow of plasma is not exactly uniform throughout the chamber, certain portions of the board become hotter than other portions, and these hot portions tend to etchback at a greater rate than the cooler portions.